1. Field of the Invention
This invention pertains to a bias circuit utilized in power amplifiers and the like for radio equipment operating in e.g. the quasi-microwave or microwave frequency band.
2. Description of Related Art
In recent years, accompanying the diversification of services offered by means of radio communications, conversion to multiband capability enabling the processing information in a plurality of frequency bands has come to be demanded of radio equipment. Together with converting radio equipment to multiband capability, the increase in the circuit size of bias circuits of power amplifiers has become a problem.
In FIG. 1, an example of a bias circuit of the power amplifier of a conventional piece of radio equipment is shown. To the gate electrode of a Field Effect Transistor 180 (below referred to as an FET), an active element carrying out power amplification, there is connected a transmission line 181 with a wavelength equal to a quarter of the wavelength λ of the used frequency, the other end of which is connected to ground via a capacitor 182. The connection point of transmission line 181 and capacitor 182 is connected to one end of a choke coil 183, the other end of choke coil 183 being connected to a DC power supply 184 generating a fixed DC voltage with respect to the ground potential.
In case capacitor 182 has a capacitance (a sufficiently high capacitance) with an impedance which becomes sufficiently small at the frequency of a transmitted signal with a wavelength λ, transmission line 181 operates as a line whose tip is short-circuited. At this point, the impedance with respect to a transmitted signal from the gate electrode of FET 180 to DC power supply 184 can be considered to be infinite. Also, the voltage of DC power supply 184 is impressed on the gate voltage of FET 180 through choke coil 183 and transmission line 181. As a result, FET 180 operates with the DC voltage of DC power supply 184 taken as a bias voltage. A bias circuit of this kind is shown in Paragraph 0005 and FIG. 6 of Japanese Patent Application Laid Open No. 11 (1999)-150431 (hereinafter referred to as Document 1).
This conventional bias circuit has had the problem that a large value had to be set for the electrostatic capacitance of capacitor 182 due to the necessity of connecting one end of transmission line 181 to AC ground. In addition, there has also been the necessity of setting a large value for the impedance of choke coil 183 in order to block AC signals communicated to DC power supply 184.
In the conventional bias circuit shown in FIG. 1 as well, by taking a line length other than a quarter of the wavelength λ of the used wavelength for transmission line 181 and considering it as a reactance element, as well as by reducing the electrostatic capacitance value of capacitor 182, it has become possible to design the combined impedance of the reactance value thereof and capacitor 182 to be infinite and to aggressively miniaturize the bias circuit.
However, since capacitor 182 normally has a random variation of 5 to 10%, there has been no means of regulating the frequency characteristics fluctuations of the bias circuit due to the random variation, so the circuit could not actually be used in mass-produced articles. In other words, the conventional bias circuit determined the operation frequency by means of a quarter-wavelength transmission line 181 which could be formed by a geometric technique having very small random variation. The capacitor 182 at that time was for AC connection to ground, and it was acceptable for the value of the electrostatic capacitance to exceed a certain value, so random variation could be disregarded.
Also, in case a conversion to multiband capability is carried out in accordance with FIG. 1, there is a need to attain the reactance of the first reactance means with a variable-reactance element, but it is assumed that a large electric currents flow in the first reactance means, so it is difficult to realize a variable-reactance element which is able to cope with large currents of that kind.
A method of using a plurality of these conventional bias circuits to respond to multiple bands is disclosed in line 4 of Paragraph 0005 and FIG. 1 of Japanese Patent Application Laid Open No. 2003-101440 (hereinafter referred to as Document 2). In the case of conversion to multiband capability, a further problem arises due to the need to use several of the aforementioned large-sized elements.
As another method of conversion to multiband capability, the method of changing the input and output impedances of the amplifier to change the matching conditions to respond to the conversion to multiband capability by varying the bias voltage of an active element is disclosed in Paragraph 0028 and FIG. 1 of Japanese Patent Application Laid Open No. 2001-267864 (hereinafter referred to as Document 3). As mentioned in lines 4 to 6 of Paragraph 0028 of Document 3, this method also assumes that a plurality of bias circuits are prepared in advance, so it has the same problem.
The problems of the aforementioned conventional technologies are summarized as follows. In the conventional bias circuit, there has been the problem that large-sized elements had to be used, so the bias circuit ended up becoming large. Also, in the conventional bias circuit, if miniaturization was attempted, there was no means of regulating frequency characteristics fluctuations due to random variation in the elements. Moreover, in the case of conversion to multiband capability, there is a need to realize the reactance of the first reactance means with a variable-reactance element, but a large electric current is assumed to flow in the first reactance means, so it is difficult to realize a variable-reactance element capable of handling large electric currents of that kind.
The present invention has been made to take these points into consideration and has for its object to provide a bias circuit which is constituted by miniature components and capable of regulating frequency characteristics fluctuations due random variation in components. Moreover, it is an object of the present invention to provide a bias circuit which can be miniaturized when converting to multiband capability.